Safety device for electric circuits



No. 625,395. Patented May 23, I899. E. J. HOUSTON & A. E. KENNELLY. SAFETY DEVICE FOR ELECTRIC CIRCUITS.

(Application filed Mar. 26, 1896.) (No Model.)

. J a My s UNITED STATES PATENT OFFICE.

EDIVIN J. HOUSTON AND ARTHUR E. KENNELLY, OF PHILADELPHIA, PENNSYLVANIA.

SAFETY DEVICE FOR ELECTRIC CIRCUITS.-

SPECIFICATION forming part of Letters Patent No. 625,395, dated May 23,1899.

Application filed March 26, 1898. Serial No. 675,345. (No model.)

To all whom it may concern.-

Be it known that we, EDWIN J. HOUSTON, a citizen of the United States, and ARTHUR E. KENNELLY, a subject of the Queen of Great Britain, residing in the city and county of Philadelphia, State of Pennsylvania, have invented certain Improvements in Safety Devices for Electric Circuits, of which the following specification is such a description as will enable those skilled in the art appertaining thereto to make and use the same, reference being had to the accompanying drawings and to the letters and figures of reference marked thereon.

The object of our invention is to protect low-pressure circuits from high pressures arisin g by accidental contacts with high-pressure circuits, so that the low-pressure circuits may be freed from danger to life or property arising from such abnormal high pressures.

Our invention consists, broadly, in connecting the low-pressure circuit to ground at one or more suitable points through a small button, layer, or prism of a chemical substance which has a high resistivity to low-pressure currents, but breaks down and becomes of low resistivity when subjected to a high electric pressure without, however, necessarily requiring the passage of a disruptive discharge.

We find that there are a number of chemical substances which possess a high resistivity to low pressures or, in other words, maintain a comparatively high degree of insulation under low pressures, but which rapidly undergo chemical decomposition and have their insulation destroyed under the application of a high pressure without, however, being necessarily subjected to the action of a disruptive discharge. In the case of most of these substances the insulation is not restored when the high pressure has been removed. Among the commercial substances which we have tried we find the best to be manganese dioxid, stannous chlorid, lead acetate, copper bichlorid, copper and ammonium chlorid, and silver sulfid. Many other salts present in a greater or less degree the same property. We employ this property in the practice of our invention by pressing a dry salt into a prism,

layer, or when convenient into a solid button of suitably small dimensions and place this mass between two electrodes,one of which is connected to ground and the other to the low-pressure circuit to be protected. Under the pressures normal to the low-pressure circuit the material practically insulates, so that the circuit is practically disconnected from the ground, only a small and almost inappreciable leakage taking place through the chemical protective device, and this only when a ground exists at some other point in the lowpressure circuit. Should the low-pressure circuit become accidentally crossed with a high-pressure circuit, the insulation of the chemical protective device or button will rapidly break down, and the low-pressure circuit will be thus automatically grounded through a comparatively low resistance. Under these circumstances the low-pressure circuit can be handled without any danger, whereas if no protective device of this nature were employed it might be fatal for a person standing on the damp ground to touch the low-pressure circuit after the cross has been established.

Figures 1 and 2 represent in longitudinal cross-section and plan a form of our chemical protective device embodying the principles of our invention. Fig. 3 shows in cross-section another form of our chemical protective device. Figs. 4,5, and 6 are diagrams of circuits, showing the connections of our protective device and its applications to the protection of low-pressure circuits.

In Fig. 1 an insulating-tube of glass, porcelain, earthenware, or other suitable material 1 1 is provided with a coarse screw-thread in its interior. 3 is a metal screw, whichmay be of brass or other good conducting mate rial, which is screwed tightly into one end of the insulating-tube, so as to plug it. 4 is adisk, layer, stance used. As represented in the figure, it is approximately one-sixteenth of an inch thick and three-eighths of an inch in diameter. A button of this size in sulfid of silver will permanently insulate under a pressure of approximately fifty volts. A metal screw 2, similar to 3, is then screwed tightly into the or button of the chemical subinsulating-tube and secured in place by the metal collar 5, thus insuring close contact with the sides of the button upon the faces of the screws 2 and 3. The device thus assembled may be supported permanently be-- tween the jaws of a metal clip, one jaw being connected to the low-pressure circuit and pressing, say, upon the head of the screw 2 and the other jaw being connected to ground and pressing on the head of the screw 3, or wires may be soldered to the heads 2 and 3 before the screws are forced into the device, or any suitable electric connection may be made to the screws 2 and 3, as by wires, and the wires led in any desired manner to the circuit to be protected and grounded, respectively.

A simpler form of chemical protective device is shown in cross-section in Fig. 3. Here two metallic springs 6 7 are rigidly supported from the face of an insulating-board 9 9 by bolts 8 S, which run to the back of the board and which are in metallic connection with the conductors 10 and 11. Conductor 10 may be run to the low-pressure circuit to be protected, and the conductor 11 to a suitable ground connection. The chemical protective button 4 is slipped into the spring-clip between the ends of the springs 6 7 and is maintained firmly in place by their pressure. Such a method of mounting and connecting our chemical protective device is especially suitable for switchboard use. tVhen it is desired to test the insulation of the low-pressu re circuit, the leakage to ground through the button at can be removed by inserting an insulatingwedge between the clips 6 and 7 and temporarily withdrawing the button.

Fig. 4 represents diagrammatically a telephone-circuit terminating on the premises of a subscriber. T represents the subscribers instrument, and 12 and 13 the two wires which connect him with the exchange or with other subscribers. Both sides of the telephone are represented in the figure as being connected to ground G through a chemical protective device d (1, although a single protective device mayin some cases be sufficient. Should the telephone circuit become accidentally crossed with a high-pressure circuit capable of sending a strong current and of injuring person or property at the telephone T, one or both of the chemical protective devices cl d will break down under the pressure and the circuit will be practically grounded through them. If fuses should be inserted in the circuit-wires 12 or 13, as at ff, one or both of these fuses will probably be blown, thereby disconnecting the subscriber entirely from the high-pressure connection; but even if no fuses were present, the telephone will be rendered harmless to touch by the direct ground connection established.

Fig. 5 represents diagrai'nmatically the connections of a step-down transformer on the premises of a consumer. 14 15 are highpressure Wires carrying an alternating curbut should a cross or contact take place between the primary and secondary coils of the transformer in such a manner as to enable a high pressure to be brought into connection with the secondary circuit the protective devices will one or both break down, and the secondary circuitwill thus automatically be grounded, or even short-circuited. The result will be a blowing of the fuses ff on the primary side,thereby disconnecting the faulty transformer from the circuit. If, however, only a single protective device be employed in the secondary circuit, protection will still be afforded by its ground connection, even if the primary fuses do not blow, since it will be impossible for any person standing on a wet floor or grounded plate to receive any appreciable pressure from the secondary circuit when grounded through the broken-down protective device. The highest pressure which he could possibly receive over andabove the secondary pressure would be that due to the drop of primary current through the resistance of the broken-down protective device, and we find that this resistance is comparatively negligible. Fig. 6 represents a similar transformer, in which the middle point of the secondaryis connected to ground G through one of our protective devices (Z. The middle point of the transformer is the point we prefer to connect to ground in this manner when it can be readily reached, as in the case of a three-wire distribution system.

There can be no disadvantage in connecting any number of our chemical protective devices with a low-pressure circuit at different points, for although the protection insured bya single device is generally perfectly satisfactory, yet that secured by a plurality may be considered more complete than that aiforded by a single device.

Different chemical substances have different degrees of sensitiveness in this property. Many substances appear to be absolutely devoid of this property at the pressures at which we have examined them. It appears that in most cases an electrolytic disassociation actually occurs, due to the passage of the current when the critical current density has been reached. By properly selecting the substance or the thickness of its layer we can determine the limiting pressure at which the protective device will break down, or the same result can be obtained by the mixture of sensitive substances or the admixture of an inert substance with a single sensitive substance or with the mixture of severalof such substances.

WVe do not limit ourselves to the particular IIO forms of chemical protective devices shown in Figs. 1, 2, and 3, since it is evident that the principles of our invention may be applied in a variety of forms.

What we claim as our invention, and desire to secure by Letters Patent, is the following:

1. As a protective device for electric circuits against accidental contacts with high electric pressures, a button, prism or layer of a chemical substance, not a perfect insulator, which when interposed between the ground and the low-pressure circuit to be protected, can only allow a negligibly small current to leak through it under the low pressure, but will electrically break down when the current strength leaking through it exceeds a certain limit under the influence of an abnormally-high pressure.

2. As a protective device for electric circuits or conductors against accidental contacts with high electric pressures, a button, prism or layer of a chemical substance which when interposed between the ground and the low-pressure circuit or conductor to be protected will allow an extremely feeble current to leak through it under the low pressure, but which gradually increasing with increase of pressure will break down electrically, and ground the low-pressure circuit or conductor as soon as the current strength leaking through it exceeds a certain limit under the influence of an abnormally-increased pressure.

3. In an electroprotective device, the combination of a ground connection, a connection to the circuit or conductor to be protected and a button, prism or layer of a sub stance which will normally permit only a negligibly small electric current to leak through it, but which under the influence of an unduly -increased pressure will permit such leak age to increase to a limit at which the button will electrically break down.

EDWIN J. HOUSTON ARTHUR E. KENNELLY.

Witnesses:

W. A. MILLET, WM. GRAY KNowLEs, 

